Separation of air



Jan. 17, 1967 c. SMIT 3,298,184

SEPARATION OF AIR Filed May 16, 1963 3 Sheets-Sheet l C 0 g ANO W4 r542,esMol/Ee I 6 A caM leEsso/e INVENTOR KE'NN 5T1 cgguL SH TH ATTORNEYSJan. 17, 1967 K. c. SMITH 3,

SEPARATION OF AIR Filed May 16, 1963 3 Sheets-Sheet 2 INV'ENTOR KENNETHCecu. M T

BY MA MD ATTORNEYS Jan. 17, 1967 K. c. SMITH 3,298,184

SEPARATION OF AIR Filed May 16, 1963 5 Sheets-Sheet 5 FIG. 3.

INVENTOR KENNETH CECIL SFHT'H 7 ATTORNEYS United States Patent 3,298,184SEPARATION OF AIR Kenneth Cecil Smith, Carshalton, Surrey, England,assignor to The British Oxygen Company Limited, London 6, England, aBritish company Filed May 16, 1963, Ser. No. 280,868 Claims priority,application Great Britain, May 29, 1962, 20,693/ 62 2 Claims. (Cl.62-13) This application relates to .a process for the separation fromair of a gaseous product containing about 60 to 70% by volume of oxygen.

Oxygen-enriched air containing about 60 to 70% by volume of oxygen is avaluable commodity for many purposes, for example in the manufacture ofsteel, and its production by the low-temperature separation of airinvolves a power consumption which is substantially lower than isrequired for the production of an equivalent output of high-purityoxygen in a conventional air separation plant.

It is an object of the present invention to provide a method forproducing enriched air containing about 60 to 70% by volume of oxygen bythe low-temperature separation of air in which the power consumption isminimised.

According to the present invention, a process for the production fromair of a gaseous product containing about 60 to 70% by volume of oxygencomprises subjecting compressed air, previously cooled to condensationtemperature and freed from condensible impurities, to a twostagerectification to produce a first liquid fraction enriched in oxygen toan extent considerably less than that required in the final gaseousproduct, partially vaporising said first liquid fraction by heatexchange with gaseous nitrogen from the first stage of the two-stagerectification to produce a second liquid fraction having an oxygencontent equal to that required in the gaseous product, and anoxygen-rich gaseous fraction, the gaseous nitrogen itself beingcondensed, using the condensed nitrogen so formed to provide refluxliquid in both stages of the two-stage rectification, recycling theoxygen-enriched gaseous fraction to the second stage of the two-stagerectification and vaporising the second liquid fraction to provide therequired gaseous product, by heat exchange with incoming a1r.

The second liquid fraction may be vaporised by heat exchange with theincoming air before the latter is subjected to rectification, or,alternatively, it may be vaporised in a partial condenser forming thefirst rectification stage, by heat exchange with air undergoingrectification therein.

The compressed air is conveniently cooled to saturation temperature andfreed from condensible impurities by heat exchange with outgoing gaseousproducts in switch exchangers, which term as used herein includes bothregen erators and reversing exchangers. The switch exchangers may bebalanced by any suitable method. A particularly convenient method is towithdraw a side-stream of air from the switch exchangers and to feed itafter suitable adjustment of its temperature and pressure, and, ifnecessary, removal of any residual condensible impurities, to one stageof the two stage rectification. For example, the side-stream of air maybe cooled by expansion in an expansion turbine, after removal of thecondensible impurities, and the expanded air stream fed directly to thesecond stage of the two-stage rectification. Alternatively, theside-strearn of air can be used to preheat a minor portion of thegaseous nitrogen leaving the first rectification, this minor portionafter preheating being expanded in a turbine to the second-stagepressure and combined with gaseous nitrogen leaving the second stage ofthe ice rectification before this nitrogen passes to the switchexchangers.

Three embodiments of the invention designed to produce enriched aircontaining 70% by volume of oxygen will now be described in more detailwith reference to the drawings accompanying the ProvisionalSpecification in which FIGURE 1 is a flow-sheet of the first embodiment;

FIGURE 2 is a flow-sheet of the second embodiment.

FIGURE 3 is a flow-sheet of the third embodiment.

Referring to FIGURE 1, air compressed to 2.85 atma. and which has beencooled and freed from water vapour and carbon dioxide is further cooledto its condensation temperature in a vaporiser 1 by heat exchange withan evaporating liquid fraction produced as hereinafter described. Theair is then fed to the lower column 2 of a two-stage rectificationsystem where it is separated into a gaseous nitrogen fraction and aliquid fraction somewhat enriched in oxygen. The liquid fraction is fedthrough an expansion valve 3 to the upper column 4 of the twostagerectification system, where it is separated into a gaseous nitrogenfraction which leaves the top of the column at 5 and a liquid fracioncontaining 48% of oxygen which collects at the bottom of the column andis thence fed to a condenser-evaporator 6 where it is partiallyvaporised to produce a liquid fraction containing 70% of oxygen and agaseous fraction containing 37% of oxygen by heat exchange withcondensing gaseous nitrogen from the lower column 2.. The liquid andgaseous fractions are separated in a separator 7 and the gaseousfraction is recycled to the upper column 4 at an appropriate point. Theliquid fraction is expanded to atmospheric pressure through an expansionvalve 8 and vaporised in exchanger 1 against incoming air to provide therequired gaseous product containing 70% oxygen.

A part of the nitrogen condensed in the condenserevaporator 6 isreturned to the top of the lower column 2 to serve as reflux therein,and the remainder is expanded through a valve 9 into the top of theupper column 4 to provide reflux liquid for this column.

A rather more elaborate scheme for the production of enriched aircontaining 70% oxygen is shown in FIG- URE 2.

In this form of the invention, air is compressed to 3.2 atma. in acompressor 10, precooled in a precooler 11, and further cooled byreturning separation products in two pairs of regenerators 12 and 13with the simultaneous removal of condensible impurities. At a pressureof about 3 atma. the main portion of the air then enters a partialcondenser 14, forming the first stage of a two-stage rectificationsystem; it passes in heat exchange with evaporating liquid product ashereinafter described and in which it is separated into a gaseousnitrogen. fraction and a liquid fraction containing 40% oxygen.

The liquid fraction is passed through an exchanger 15 in heat exchangewith gaseous nitrogen leaving the second stage of the rectificationsystem as hereinafter described and is then expanded through a valve 16into an adiabatic column 17 forming the second stage of therectification system, and in which it is separated into a gaseousnitrogen fraction leaving the top of the column 17 and a liquid fractioncontaining about 51% oxygen which collects at the bottom of the column.

A minor part of the gaseous nitrogen fraction leaving the top of thepartial condenser 14 is preheated in a turbine pre'heater 18 by heatexchange with air withdrawn as a sidestream from the regenerators 12 and13 and is then expanded with the performance of external work in aturbine 19 to a pressure of 1.2 atma. The exhaust from the turbine joinsthe waste nitrogen leaving the top of the column 17.

The remainder of the gaseous nitrogen fraction leaving the partialcondenser 14 is condensed in a condenserevaporator 20 by heat exchangewith the liquid fraction containing about 51% oxygen from the bottom ofthe adiabatic column 17. A part of the condensed nitrogen is returned tothe partial condenser 14 to act as reflux liquid therein, whilst theremainder after passing in heat exchange with gaseous nitrogen leavingthe column 17 in an exchanger 21 is fed through an expansion valve 22into the top of the adiabatic column 17 to provide reflux liquid forthis column.

The partially-vaporised oxygen-rich fraction leaving thecondenser-evaporator 20 is separated in an equaliser 23 into a vapourfraction containing 37% oxygen which is returned to the adiabatic column17 through a line 24, and a liquid fraction in equilibrium with this andcontaining 70% oxygen.

This liquid fraction is slightly expanded through a valve 25 into thepartial condenser 14 where it is completely vaporised to provide thegaseous product containing 70% oxygen, and from which it is withdrawnfor use, after giving up its cold content to incoming air in theregenerator 12.

Yet another alternative method is shown in FIGURE 3, which differs fromFIGURE 2 in that the side-stream from the regenerators passes through acarbon dioxide absorber 26, then through the turbine 19, and enters theadiabatic column 17. In this modification, 2025% of the entering air isbled from the columns and passes via the carbon dioxide absorber 26 andturbine 19 to the adiabatic column 17, and 75-80% of the air passes tothe partial condenser 14, as in the embodiment shown in FIGURE 2.

The above examples are of only three embodiments of the invention, andother embodiments are possible; for example, a three regenerator systemcould be used instead of the two regenerator system shown in FIGURES 2and 3, and other methods of balancing the regenerators could be used,such as the so-called Trumpler pass method,

I claim:

1. A process for the production from air of a gaseous product containingabout 60 to 70% by volume of oxygen comprising the steps of subjectingcompressed air, previously cooled to its saturation temperature andfreed from carbon dioxide and water to a first stage rectification toproduce a first liquid fraction enriched in oxygen and a first gaseousnitrogen fraction, subjecting said first liquid fraction enriched inoxygen to a second stage rectification to produce a second liquidfraction, further enriched but to an extent considerably less than thatrequired in the final gaseous product, and a second gaseous nitrogenfraction, withdrawing the second liquid fraction from the second stagerectification zone and passing the entire second liquid fraction in heatexchange with at least a part of said first gaseous nitrogen fraction topartially vaporize said second liquid fraction and produce a thirdgaseous fraction having a higher oxygen content than air and a residualliquid fraction having an oxygen content equal to that required in thegaseous product, at least a part of said first gaseous nitrogen fractionitself being con densed to provide condensed nitrogen, using saidcondensed nitrogen to provide reflux liquid in both the first and thesecond stage rectification, recycling said third gaseous fraction tosaid second stage rectification, and vaporizing said residual liquidfraction to provide the required gaseous product by heat exchange withincoming air, cooling at least a part of said compressed air tosaturation temperature and freeing at least a part of said compressedair'from condensible impurities by heat exchange with outgoing gaseousproducts in switch exchangers, withdrawing compressed air as aside-stream from said switch exchangers and subsequently feeding thecompressed air to a stage of said first stage rectification, preheatinga minor part of the first gaseous nitrogen fraction by heat exchangewith said air Withdrawn as a sidestream from said switch exchangers,said minor part being expanded in an expansion turbine and combined withsaid second gaseous nitrogen fraction, said air withdrawn as aside-stream from said switch exchanger leaving said heat exchange stepbeing combined with the main stream of compressed cooled air leavingsaid switch exchangers. 2. A process for the production from air of agaseous product containing about 60-70% by volume of oxygen comprisingthe steps of subjecting compressed air, previously cooled to itssaturation temperature and freed from carbon dioxide and water to afirst stage rectification to produce a first liquid fraction enriched inoxygen and a first gaseous nitrogen fraction, subjecting said firstliquid fraction enriched in oxygen to a second stage rectification toproduce a second liquid fraction, further enriched but to an extentconsiderably less than that required in the final gaseous product, and asecond gaseous nitrogen fraction, withdrawing the second liquid fractionfrom the second stage rectification zone and passing the entire secondliquid fraction in heat exchange with at least a part of said firstgaseous nitrogen fraction to partially vaporize said second liquidfraction and produce a third gaseous fraction having a higher oxygencontent than air and a residual liquid fraction having an oxygen contentequal to that required in the gaseous product, at least a part of saidfirst gaseous nitrogen fraction itself being condensed to providecondensed nitrogen, using said condensed nitrogen to provide refluxliquid in both the first and the second stage rectification, recyclingsaid third gaseous fraction to said second stage rectification, andvaporizing said residual liquid fraction to provide the required gaseousproduct by heat exchange with incoming air, cooling at least a part ofsaid compressed air to saturation temperature and freeing at least apart of said compressed air from condensible impurities by heat exchangewith outgoing gaseous products in switch exchangers, withdrawingcompressed air as a side-stream from said switch exchangers afterremoval of residual condensible impurities therein and subsequentlyexpanding said compressed air in an expansion turbine and feeding theexpanded air to the second stage of rectification.

References Cited by the Examiner UNITED STATES PATENTS 2,513,306 7/1950Garbo 62-29 X 2,560,469 7/1951 Ogorzaly 6214 2,572,933 10/1951 Houvener62-14 2,648,205 8/1953 Hufnagel 6213 2,655,796 10/1953 Rice 62142,688,238 9/1954 Schilling 6229 X 2,709,348 5/1955 Yendall 62-142,955,434 10/1960 Cost 6213 2,996,890 8/1961 Baldner 6 214 3,066,49412/1962 Potts 6213 X 3,108,867 10/ 1963 Dennis.

3,113,854 12/1963 Bernstein 6229 3,127,260 3/1964 Smith 6229 X NORMANYUDKOFF, Primary Examiner.

1. A PROCESS FOR THE PRODUCTION FROM AIR OF A GASEOUS PRODUCT CONTAININGABOUT 60 TO 70% BY VOLUME OF OXYGEN COMPRISING THE STEPS OF SUBJECTINGCOMPRESSED AIR, PREVIOUSLY COOLED TO ITS SATURATION TEMPERATURE ANDFREED FROM CARBON DIOXIDE AND WATER TO A FIRST STAGE RECTICICATION TOPRODUCE A FIRST LIQUID FRACTION ENRICHED IN OXYGEN AND A FIRST GASEOUSNITROGEN FRACTION, SUBJECTING SAID FIRST LIQUID FRACTION ENRICHED INOXYGEN TO A SECOND STATGE RECTIFICATION TO PRODUCE A SECOND LIQUIDFRACTION, FURTHER ENRICHED BUT TO AN EXTENT CONSIDERABLY LESS THAN THATREQUIED IN THE FINAL GAESOUS PRODUCT, AND A SECOND GASEOUS NITROGENFRACTION, WITHDRAWING THE SECOND LIQUID FRACTION FROM THE SECOND STAGERECTIFICATION ZONE AND PASSING THE ENTIRE SECOND LIQUID FRACTION IN HEATEXCHANGE WITH AT LEAST A PART OF SAID FIRST GASEOUS NITGROGEN FRACTIONTO PARTIALLY VAPORIZE SAID SECOND LIQUID FRACTION AND PRODUCE A THIRDGASEOUS FRACTION HAVING A HIGHER OXYGEN CONTENT THAN AIR AND A RESIDUALLIQUID FRACTION HAVING AN OXYGEN CONTENT EQUAL TO THAT REQUIRED IN THEGASEOUS PRODUCT, AT LEAST A PART OF SAID FIRST GASEOUS NITROGEN FRACTONITSELF BEING CONDENSED TO PROVIDE CONDENSED NITROGEN, USING SAIDCONDENSED NITROGEN TO PROVIDE REFLLUX LIQUID IN BOTH THE FIRST AND THESECOND STAGE RECTIFICATION, RECYLING SAID THIRD GASEOUS FRACTION TO SAIDSECOND STAGE RECTIFICATION, AND VAPORIZING SAID RESIDUAL LIQUID FRACTIONTO PROVIDE THE REQUIRED GASEOUS PRODUCT BY HEAT EXCHANGE WITH INCOMINGAIR, COOLING AT LEAST A PART OF SAID COMPRESSED AIR TO SATURATIONTEMPERATURE AND FREEING AT LEAST A PART OF SAID COMPRESSED AIR FROMCONDENSIBLE IMPURITIES BY HEAT EXCHANGE WITH OUTGOING GASEOUS PRODUCTSIN SWITHC EXCHANGERS, WITHDRAWING COMPRESSED AIR AS A SIDE-STREAM FROMSAID SWITCH EXCHANGERS AND SUBSEQUENTLY FEEDING THE COMPRESSED AIR TO ASTRAGE OF SAID FIRST STAGE RECTIFICATION, PREHEATING A MINOR PART OF THEFIRST GASEOUS NITROGEN FRACTION BY HEAT EXCHANGE WITH SAID AIR WITHDRAWNAS A SIDESTREAM FROM SAID SWITCH EXCHANGERS, SAID MINOR PART BEINGEXPANDED IN AN EXPANSION TURBINE AND COMBINED WITH SAID SECOND GASEOUSNITROGEN FRACTION, SAID AIR WITHDRAWN AS A SIDE-STREAM FROM SAID SWITCHEXCHANGER LEAVING SAID HEAT EXCHANGE STEP BEING COMBINED WITH THE MAINSTREAM OF COMPRESSED COOLED AIR LEAVING SAID SWITCH EXCHANGERS.